WP3 : Aquifer Systems and Multiphasic Context

Mohamed Azaroual (BRGM), Lionel Mercury (University-ISTO)

Background and rationale

This WP of the Labex Voltaire focusses on hydrosystems along the first fictive bark (0 – 5 km) beneath Earth surface, bridging the deeply rooted Earth crust processes treated in WP2 with the quicker and short-lived geological cycles of shallow aquifers and soils. WP3 activity combines field, experimental, and numerical modelling approaches.

Aquifers are hydraulic perpetual flow systems, as well as chemical and thermal flow systems, on very extensive spatial and temporal scales. In addition, their exploitation can generate topological changes in the host rock, conducive to redistributing the mechanical stress fields. Finally, surface aquifers interact with short cycles of supergene and atmospheric environments, themselves marked to various extents by anthropization. They are then subject to the influences (if not to the control) of the living component.

Two characteristics define the project group: seeing the aquifer as a vadose zone–saturated zone continuum and looking for the driving forces of flows and water-rock interactions, to be able to quantitatively predict the behavior of hydrogeosystems. The group also uses the common partitioning by spatial scales (Fig. 1), but fundamentally bears on an explicit distinction between discrete vs. continuum representations. Whichever the scale, the discrete representation allows to describe (the most quantitatively possible) the mechanistic “local” transfer events, while the continuum representation is adapted to visualize (again as quantitatively as possible) how these mass and heat flows establish transient/permanent regimes.

Figure 1: Illustration depicting different spatial scales, spanning roughly 4 orders of magnitude and organizational levels, across which the distinction is on discrete mechanistic representation vs. continuum ‘homogeneized’ laws

Regarding the study of the superficial vadose zone, our goal is to develop thermodynamic modeling of capillary action, especially with respect to the chemical and mechanical effects that it can generate, in parallel with the study of hysteresis phenomena and their consequences on the characteristic volumes of capillary water or air on the pore scale.

This activity involves the development of adapted experimental setups, either for measurements of natural material properties under constrained conditions, or on analogous materials for quantitative understanding of causes and effects. It also corresponds to the development of calculation tests in 1D and 2D chemistry-transport, integrating the results of prior studies.

Finally, the association of geochemistry and geomechanics is developed, especially the role of capillary traction in compaction and micro-cracking. With regard to the deep vadose zone, the understanding of the physicochemical role of the biosphere is of major importance, notably for the comprehension of redox disequilibria conditions and thus the geochemical stability domains of exploitable compounds (gas hydrates, for example).

A second important point is the quantitative description of brine excess properties in the multiphasic context, particularly for deep storage (ie CO2) and geothermal energy production. Furthermore, in these contexts, one finds the importance of chemical-mechanical associations, for example, with the concurrent effects of clogging and capillary microcracking during CO2 injection phases.

Regarding the physicochemistry of natural systems, the effort is on evaluating the respective weight of surface effects and volume effects when the characteristic size of the porosity or grain material becomes less than one micron. A key question pertains to the clear definition of the size thresholds where the behavior of objects changes significantly. The role of dissolved species, often bearing an (unlimited) electrical field, is of critical importance for a solution trapped in limited pore spaces, especially as for the scope of what is called “confinement”. Once the confinement threshold(s) is(are) well understood, these fundamental studies can be applied to suitable aquifers using the simple criteria of the size of the average or localized porosity.

This WP has a methodology closely associating a better understanding of where and how the main gradients accumulate (special “small” scale, local micro-environment = mechanistic approach) with a relevant  description of the flow dissipating these gradients (continuum scale). Conceptually, this methodology consists in the reaction path approach, making use of mass balance and thermodynamic equilibrium (even partial equilibrium when including kinetics) to mechanistically describe the geochemical routes of a system from the initial to final states.

  • The group try to gather both geochemical and hydrological clues of local disequilibrium situations, so as to infer the leading parameter(s) and process(es).The main effort consists then in simulating process(es) and condition(s) assumed to drive a deviation out of the equilibrium to accumulate a driving force without immediate dissipation.
  • In a second step, classical modeling are harnessed to predict how the transport laws may dissipate or “homogeneize” these local gradients in the downward flowlines and/or at the upper scale including the capillary fringe ensuring the transition between the unsaturated (vadose) zones and saturated (aquifer).

Results of WP 3 (2011-2014)

Studies at the scale of the Unsaturated Zone – Saturated Zone: We confirmed the interest of geophysical studies to obtain a 3d picture of the pedological bodies and their functioning. This type of non-intrusive sensors is one for the key tracks that the Labex Voltaire targets to develop in the near future. This study was a relevant support for the PIVOTS project application, offering promising results for geophysical tools in environmental monitoring. In particular, the importance of crossing the techniques and coupling the data inversion are on the core of this new project, part of the Voltaire2 dynamics (see project 2020-2024).

Figure 2. Proof of Concept illustration supporting what is called a geophysical taxonomy of soils hydrous properties as a function of depth (Buvat et al., Geoderma, 2014).

A modelling effort was led to include colloids, especially the nano-particles growingly disseminated and/or used in the environment, into the mass balances at the Unsaturated Zone and Saturated Zone scales. This modelling project used the Pore Numerical Model (PNM) approach to quantify the role of the aggregation and attached processes on the nanoparticles (NPs) transport (Sameut et al., 2011).

Studies at the pore scale: An experimental study was conducted on the properties of liquids trapped in pores and channels. We have successfully designed a partition function to translate IR spectra into thermodynamic properties, (Bergonzi et al., 2014 PCCP). In terms of breakthroughs, measurements in fluid inclusions (each is analogous to one closed pore) demonstrated that the reactivity in the first micrometer layer close to the solid-liquid interface, was larger than in the bulk (Bergonzi et al., PCCP, 2016; see highlights). The interactions with researchers working at the “MESA+ Institute for Nanotechnology” (Twente, The Netherlands) opened new possibilities for quantifications of pore/channel processes fabricating geological labs on a chips (GLoCs) materials by derivation of nano-technology techniques.

Using nano-channels etched in silicon-silicon wafers in the 5 nm-100 nm range, we have demonstrated that the geochemical confinement is restricted to 30 nm high (or less) channels, in contradiction to the fluid inclusions studies.

Figure 3. Synchrotron-based FTIR records of the Gibbs free energy of liquid water trapped at various nanoscale thresholds (Bergonzi Ph. D., 2015).

One PhD was launched taking benefit of the PNM tool developed for nano-colloids transport,  to study the reactive transport of nitrates in saturated and unsaturated conditions. This study was also the chance to modify the classic approaches in PNM and involves the “links” in the reactive system, normally restricted to the “pores”.

Another PhD, fully supported by Voltaire, was designed to clarify the scale-dependent FTIR results, focusing this time on the consequences of confinement in terms of phase transitions. The basic idea was to use heterogeneous materials associating macro- and nano-porosities to see whether the nanoscale geochemical signatures was able to imply macroscale (and visible) mass transfer.

The field scale was pursued with a PhD about the preferential transport in soils, as a follow-up of the geophysical study, and to prepare the PIVOTS project (man-sized, borehole along a 25m thick unsaturated zone). Also at this scale, a post-doc worked during two years to implement the effect of capillarity on the pollutant migration in the critical zone.

Results of WP 3 (2015-2018)

In line with the targeted objectives during the 2011-2014 period, WP3 developed an original experimental platform for studying the finite size effect on water-rock interactions, taking benefit on fluid inclusions and nanofluidic silicon-based wafers as model pores under controlled conditions and infilling liquids. Quantitative information are obtained through Voltaire-funded instruments combining high resolution optical microscopy with micro-beams spectroscopies (Figure 4).

Figure 4. Nanofluidic reactive platform to study surface and volume effects of pores on the mobility and reactivity of trapped fluids, inside model pores

Based on the platform completed by beamtimes at Soleil and ESRF synchrotrons, WP3 succeeded to record various confinement-type signatures in water-saturated pores, offering a strikingly modified picture on the water-rock interactions mechanisms at (large) pore scale (Bergonzi et al., PCCP, 2016; see Highlights; Mercury et al., JPC-C, 2016; Baum et al., Procedia 2017). The more recent advance (Fig. 5) is combining geochemistry and poromechanics, to demonstrate how drying conditions may impact the local stress fields (up to crystal fracturing) around a pore in which water is subjected to strong capillary conditions.

Figure 5: Edges-controlled fracturing in pores bearing water maintained at -130 MPa, and Raman shifts recorded in the quartz solid host, at the optical plane of the inclusion and 10 µm above. From : Mercury et al., J. Geophys. Research, submitted).

Beyond the one-pore scale, the effect of heterogeneity on mass transfers was studied through modeling with a post-doc (Nsir et al., JCH, 2018), and experiments via ISTO-CEMHTI collaboration (Fig. 5) and a Voltaire PhD (Hulin’s Ph. D. 2017; 2 papers submitted to GCA).

In all water-rock modelling, respecting the natural composition of aqueous solutions is highly needed, especially when brines are involved. A thermodynamic framework, based on excess properties and water activity, was developed within the Pitzer model to drastically extend the intervals of concentration tractable by this approach (Lach et al., 2015, J. Sol. Chem.; Lassin et al, 2015, Am. J. Sci.; Lach et al, 2016, Comp.&Geosc., Fig 6).

Figure 6. Experiments-consistent solubilities of various crystal phases predicted up to very high concentrations (left: Lassin et al., 2015; right: Lach et al., 2015).

In terms of continuum scale in the field, WP3 works enabled to obtain a major funding (1.2 M€) from region Centre, consisting in a man-sized borehole, heavily equipped with various (geophysical, thermal, biogeochemical) sensors for in situ long-term monitoring of water and chemicals fluxes at the unsaturated zone scale. This “O_ZNS observatory” (Fig. 7) is one of the platform of the PIVOTS project (12 M€) led by BRGM as a major regional research infrastructure dedicated to the environmental monitoring operations run by the Earth Sciences institutes located in region Centre. The chosen site is a 25 m thick unsaturated zone (UZ) in an heterogeneous multilayered sediment, associating low permeable marl formations with fissured limestones, and contaminated for years by diffuse chemicals.

Figure 7. Sketchy illustrations of the future O-ZNS borehole, to be drilled during spring 2019, and put in operation towards the end of 2019